Key to the promise of stem cells for therapy is a method for non-invasively detecting cellular migration and differentiation. MRI-based cell tracking using magnetic particles has been utilized to detect cell migration in vivo, however, it i currently incapable of detecting cell differentiation. This is because the magnetic particles currently used for cell tracking create the same MRI contrast whether they are inside a stem cell or mature cell. Here we propose to design, synthesize and characterize novel classes of biopolymer encapsulated metallic nanoparticles which will enable the use of MRI to non-invasively detect stem cell differentiation, in vivo. The underlying principal of these new particls is the ability of these particles to achieve large enhancements in molar relaxivity based upon enzyme-triggered modification of the biopolymer coat. Briefly, the MRI properties of magnetic particles vary greatly depending on the coating thickness over the magnetic cores and their aggregated state, even though the amount of magnetic material remains constant. Additionally, MRI contrast agents have vastly different properties based on water solubility. We will synthesize different classes of biopolymer coated metallic core particles whose coating will be able to be enzymatically cleaved and removed. The biopolymer coatings will be biocompatible, yet highly resistant to passive degradation in the cells. Furthermore, for some particles, the metallic core will be able to dissolve. Dynamic manipulations of these phenomena will result in large enhancements of relaxivity and hence, of the MRI signal. Computer simulations predicting relaxivity changes as a function of coating thickness will guide the nanoparticle fabrication. We will then test whether enzyme-mediated removal of the biopolymer coating indeed results in modulation of the MRI properties. Simulations predict a 10-fold increase in r2, a 4-fold increase in r2* for iron oxide based particles, and a 50-fold increase in r1 for manganese based particles. These new classes of particles will form the technological core for non-invasive visualization of stem cell differentiation in intact organisms by MRI and can be further generalized to monitoring gene expression. Upon internalization into cells, nano- and microparticles are sequestered in endosomes and lysosomes. While it has been known that dextran coated magnetic particles slowly degrade within these structures, no study has yet purposely harnessed this phenomenon. The innovation of this proposed work is that we are utilizing the chemical environment within these subcellular structures as a medium for enzymatic reactions that will modulate water accessibility to the metallic cores, and in some cases, to dissolve them. Future implementation of these particles will utilize reporter enzymes to react with particles, engineered as transgenes, which will be expressed during the transition from stem cell to mature cell. So, rather than particles slowly degrading over several weeks, we will force the decomposition of these constructs to occur within hours once triggered, thus providing a relatively rapid, non-invasive, high resolution, three dimensional readout of stem cell differentiation.